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I have a set of IR LEDS, and two different cameras. The resulting image from one of them is purple-ish, the other is grayscale when blocking all visible light with a thick plastic bag. Both cameras have different sensors. Does anybody have a good technical source to read about this or could provide an explanation?

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    $\begingroup$ Hm, I think you already know your answer: the color (sub)pixels in the one camera where your infrared seems to be colorless are uniformly sensitive to IR, and in the other camera, whatever pixel's colors/color filters correspond to purple (blue and red, I'd guess) are more responsive to IR. You don't need a special explanation for this phenomenon – you'll want to read up on how color camera sensors work, in general. $\endgroup$ Nov 15, 2022 at 14:24
  • $\begingroup$ Does this mean that the sensor giving purple-ish color is cheaper and maybe even not meant to be used for night vision @MarcusMüller? $\endgroup$
    – Mikel B
    Nov 16, 2022 at 8:13

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Assuming it's a typical silicon focal plane array, then the silicon itself is inherently monochrome, with a responsivity to light that you can look up on the web.

Unless your camera is insanely expensive*, color imaging works by applying colored filters over each pixel -- do a web search on "Bayer pattern" for the most common ones.

In the case of the camera that comes out looking grey, the filters are all passing IR light equally, with a result that adds up to a grey image. In the case of the camera that comes out looking purple (which is what my cell phone does with 940nm LEDs), the green filter passes less IR light than the red or the blue -- so you end up with a purplish image.

* Insanely expensive color cameras use three chips, and a three-way beamsplitter. I'm not even sure if this technology is still in use -- 20 years ago it was the standard for broadcast-quality TV cameras; I don't know for sure if it's in use now.

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  • $\begingroup$ Would you say the camera sensor yielding purple-ish images are not meant for night vision but the other is @TimWescott? $\endgroup$
    – Mikel B
    Nov 16, 2022 at 8:11

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